Could be so much better, but with a world as rich as One Piece to work with, it's still awesome.
You can raise your lvl and buy the special attacks of each character, like an rpg.
Also you need to play this mode to unlock the characters.
This is nice and all, but the main aspect of this game are the battles themselves, they are dynamic and so much fun.
You can attack,block,jump,and carry objects; and even thought you just have one button to attack, once you get used to it you realize it's full of possibilities. Almost every attack, if not all of them, comes from the anime/manga, which is awesome for a fan.
You can use your special bar either to dodge, do special attacks, or go rage mode where your power increases and your health keeps decreasing
Dodging is also beautifully implemented, you move trespassing everything in your way, but each character has a proper animation.
Luffy does the gear 2nd soru, zoro rolls, kuma teleports, ussop runs like a coward, etc...
All those little things make this game really enjoyable for fans.
And last but not least, since I don't think gamespot is gonna give this a review, the playable characters:
Luffy.
Zoro.
Sanji.
Nami.
Ussop.
Chopper.
Hancock.
Jinbei.
Buggy.
Ivankov.
Ace.
Kuma.
Crocodile.
Mihawk.
Magellan.
Whitebeard.
Aokiji.
Kizaru.
Akainu.
Blackbeard.
Then there's like 40 assist characters that do something when you call them and leave.
My only complaint would be that Franky is not a playable character, just an assist. What the hell?!?!
All in all, if you like One piece, get this in any way possible.